Many modern communications systems employ a twisted wire pair using differential signaling to transmit data. Among the communications systems in this category are telecommunications systems such as the various types of Digital Subscriber Line (xDSL), and other digital carrier systems. xDSL may include, for example, asymmetric digital subscriber line (ADSL), asymmetric digital subscriber line two plus (ADSL2+) and very high-speed digital subscriber line (VDSL) systems.
In ADSL2+ modems with a frequency range of 138 kilohertz (KHz) to 2.2 megahertz (MHz), the signal-to-noise ratio (SNR) is often degraded by the presence of radio and other unwanted signals that are inadvertently picked up by the system. In particular, AM radio signals in the range of 540 KHz to 1.6 MHz may cause significant interference. In VDSL modems, with an upper frequency of 12 MHz to 17 MHz, there are even more disturber sources that can corrupt the SNR.
These unwanted signals are impressed on the twisted pair line as a common-mode signal with respect to ground. In conventional xDSL modems, receivers are designed to accept differential signals and reject common mode signals. The modems typically include a common mode filter to reject a substantial portion of the common mode signal. Depending on the quality and balance of the twisted pair line, some portion of the common-mode signal may be converted to a differential signal in the line itself. Under typical conditions, this portion may be enough to limit system performance.
Once converted to a differential signal by any means, the disturber signal appears as noise mixed with the intended communication signal and this effectively degrades the SNR and hence the data throughput performance of the modem. If the common-mode noise signal Y is known independently of the signal X+Y that contains both noise signal Y and differential communication signal X, then it is possible for the modem, using digital signal processing (DSP) means, to subtract the signal Y from the signal X+Y and be left with just the signal X. In other words, it is possible to uncover the intended communication signal in the presence of the common-mode noise signal.
In order to support DSP cancellation of the common-mode signals in an xDSL modem, two additional functional blocks are required in hardware: (1) a second receiver input containing an analog-to-digital converter (ADC); and (2) a circuit to generate a common-mode reference signal which contains substantially only the common-mode content of the line.
FIG. 1 illustrates a conventional common-mode reference signal generation circuit. In FIG. 1, the noise estimate is based on a common-mode reference noise signal, which is sampled via an additional winding on the magnetic core of the common mode filter inductor that couples the input lines to the receiver. Existing communication standards require that the primary winding or windings of the line transformer or filter inductor be isolated from chassis ground and from the secondary winding. A common-mode reference signal detector must span and yet provide galvanic isolation between the telecom network voltage (TNV) circuitry and the safe effective low voltage (SELV) circuitry. The breakdown voltage of this isolation must be at least 1500 volts of alternating current (VAC). Therefore, a common-mode reference noise signal, either from the center tap of the primary winding of the line coupling transformer or from an additional winding on the common mode filter inductor (as shown in FIG. 1) cannot be connected directly to the radio frequency (RF) noise canceller. Isolation by means of and an additional and more costly high-voltage transformer is required.